Process for forming a chromium oxide insulating layer between the pellets and the cladding of a nuclear fuel element, and fuel element having such an insulating layer

Abstract

An organometallic chromium compound in the gaseous phase is brought into contact with a substrate consisting of the inner surface of the zirconium alloy tubular cladding (5) of the fuel element, or the outer surface of the pellets of fuel material, the substrate being kept at a temperature between 300° and 600° C. The organo metallic compound may consist of chromium acetylacetonate. The process enables a chromium oxide coating to be obtained inside the tubular cladding (5) and/or on the outer surface of the nuclear fuel pellets. Pellet/cladding interaction is thus prevented or limited when this fuel element is used in the reactor.

We claim: 
     
       1. A process for producing a chromium oxide insulating layer between pellets and a cladding (5) of a fuel element for a nuclear having a zirconium alloy cladding (5) and a stack of pellets of sintered fuel material which are introduced into the cladding (5) during manufacture of the fuel element, comprising the steps of bringing an organometallic chromium compound into contact, in gaseous form, with at least one substrate consisting of the inner surface of the cladding (5) and/or the outer surface of the fuel pellets, in the presence of an oxidizing gas, and maintaining said substrate at a temperature between 300° and 600° C. in order to produce chemical vapor deposition of chromium oxide. 
     
     
       2. The process as claimed in claim 1, wherein the organometallic chromium compound is chromium acetylacetonate (C 5  H 7  O 2 ) 3  -Cr, and wherein the temperature of the substrate is between 350° and 600° C. 
     
     
       3. The process as claimed in claim 2, wherein the chromium acetylacetonate is heated to a temperature between 180° and 200° C. before being brought into contact with the substrate. 
     
     
       4. The process as claimed in claim 1, wherein the vapor of the organometallic chromium compound is entrained by a vector gas ensuring its contact with the substrate. 
     
     
       5. The process as claimed in claim 4, wherein the vector gas is argon. 
     
     
       6. The process as claimed in claim 4, wherein the oxidizing gas consists of oxygen mixed in a given proportion with the vector gas and with the organometallic compound vapor. 
     
     
       7. A fuel element for a nuclear reactor having a tubular zirconium alloy cladding (5) and a stack of pellets of sintered fuel material arranged inside the cladding (5), comprising at least one chromium oxide insulating layer less than 10 μm thick inserted between the inner surface of the cladding (5) and that outer surface of the pellets situated facing the inner surface of the cladding (5). 
     
     
       8. The fuel element as claimed in claim 7, wherein the insulating layer is deposited on the inner surface of the cladding (5). 
     
     
       9. Fuel element as claimed in claim 7, wherein the insulating layer is deposited on the outer surface of the pellets of fuel material. 
     
     
       10. The fuel element as claimed in claim 7, wherein the insulating layer consists of both a coating deposited on the inner surface of the cladding (5) and of a coating deposited on the outer surface of the pellets. 
     
     
       11. The fuel element as claimed in claim 7, wherein the cladding (5) is coated on its outer surface with a chromium oxide layer.